The molecular mechanisms through which cells and neuronal growth cones migrate, adhere to the substrate and find their targets are varied, appear to often work in redundant pathways, and appear to be conserved in evolution. Although many molecules have already been identified and studied, especially using the invertebrate model systems, it is clear that many other molecules involved in these processes still need to be discovered. Kallmann syndrome (KS) is a human inherited disorder defined by the association of anosmia and hypogonadism. The human gene responsible for the X-linked form of the disease has been identified. The chicken, the quail and recently two zebra fish homologs, but not the mouse one have also been identified. These genes code for novel secreted proteins, KALs, which contain a new combination of domains: the four-disulfide-core domain together with fibronectin type III domains. The vertebrate genes are expressed in the olfactory bulb and are required for proper neuronal connectivity and cell or axonal migration in this region both during development and in adult life. Studies in the chicken suggest that the KAL proteins may be new components of the mechanisms of cell/axon migration or cell adhesion. A C. elegansgene, identified by the Sequencing Consortium, has significant homology to the human gene for X-linked Kallmann syndrome. The gene, which we have named
kal-1, maps on LGI near
unc-54. A cDNA clone from the Kohara collection has been obtained and sequenced. Comparison with the genomic sequence shows that the mRNA is composed of 6 exons. The open reading frame codes for 700 aa. The domain topology of the protein, is conserved between nematode and vertebrates. The main difference is the presence at the C-terminus of the C. elegansprotein of a stretch of hydrophobic residues. We have studied the expression of
kal-1 with the use of a series of lac-Z and GFP reporter constructs. The reporter experiments indicate that 6-8 cells begin expressing the gene in 200-cell-stage embryos. In larval stages three groups of cells express the gene. A group of neurons (15-20) in the nerve ring region surrounding the pharinx. A group of 4-5 neurons in the tail ganglia. The two canal associated neurons (CANs) at mid body. In adults the pattern does not change but it is more restricted. Sequences upstream of the ATG and in the first intron appear to have a role in modulating expression. We have isolated a deletion mutant,
kal-1(
gb503), using TMP + UV as mutagen and screening by PCR. The deletion is 2.2 Kb long and eliminates the 440 terminal residues. RT-PCR analysis of the mature mRNA present in the mutant strain suggests that the mutant is a functional null. The mutant shows no major visible phenotype. More subtle defects present at low penetrance in the mutant will be discussed. To help identify the function of the gene we have injected worms with constructs overexpressing either the full length or truncated forms of the KAL-1 protein fused to GFP. Worms transformed with these constructs show various abormalities both during embryogenesis and in larval development. An interpretation of these phenotypes will be presented. We have also identified and sequenced cDNAs with significant homology to Kal genes in Drosophilaand in the silk worm. The N-terminal regions of the predicted proteins are strongly conserved but the insect proteins are shorter than the nematode and vertebrate ones. By in situ hybridization the Drosophila gene appears to be expressed in embryos in the stomatogastric ganglia. Thus also in this system the homolog of the Kallmann syndrome gene appears to be expressed by neurons.